/**
 * @param {number[]} nums
 * @return {number}
 */
var missingNumber = function (nums) {
  if (!nums || !nums.length) return 0;
  let left = 0;
  let right = nums.length - 1;
  while (left < right) {
    let mid = left + ~~((right - left) / 2);
    if (nums[mid] !== mid) {
      right = mid;
    } else {
      left = mid + 1;
    }
  }
  return nums[left] === left ? nums.length : left;
};
